This invention relates to resin compositions and precision sliding parts made of such resin compositions used for sliding portions of carriages mounted on printers, optical or magneto-optic electronic image memory/reading devices or memory devices used with MO, DVD or CD-ROM, and precision sliding parts of other electronic devices.
For printers and optical or magneto-optic image memory/reading devices (or scanners), which are input/output devices for computers, and memory devices used with MO, DVD and CD-ROM, it is necessary to smoothly move and accurately position scanning parts or printing heads, or optic or magneto-optic pickups. Devices for smoothly moving them along an orbit are called carriages.
For sliding parts such as carriages or precision sliding parts of electronic devices, extremely high dimensional accuracy and stable sliding properties at normal temperature are required. It is also required that they can perform precise positioning and smooth movement.
For precision sliding parts made of synthetic resin, it is required that they have precision molding properties including low molding shrinkage rate and small molding strain, and that sliding parts and mating parts are both usable in low wear amounts. As conventional resin materials for such sliding parts, those are known in which modified polyphenylene ether, polybutyleneterepht halate or polycarbonate is used as a main component and glass fiber and tetrafluoroethylene (PTFE) resin are added to increase precision molding properties.
In a resin material for forming precision sliding parts of electronic devices, PTFE is added to improve lubricity. But it is difficult to sufficiently stabilize with only PTFE the sliding properties including low friction coefficient. Normally, grease and lubricating oil (hereinafter referred to as grease and the like) are further applied to the sliding surface to give it required sliding properties.
For precision sliding parts made of synthetic resin for electronic devices, their mating member is made of stainless steel or steel whose surface is treated by electric nickel plating. They are metallic materials which are relatively low in hardness.
A conventional resin composition for precision sliding parts, which is used with grease and the like, due to scattering and evaporation of lubricant, may pollute the environment and the inside of the device.
Also with the conventional resin composition for precision parts, which is used grease and the like, dust tends to stick to the sliding portion. If dust invades the sliding surface, the mating member and the bearing portions may be worn by dust.
Also, in a conventional resin composition for precision sliding parts, which is used with grease and the like, if maintenance is neglected, lubricant may become scarce. This may cause abnormal sound due to shortage of lubricant.
With precision molded bodies comprising a conventional lubricative resin, it is difficult to sufficiently stabilize their sliding properties. The fibrous reinforcing material added so that they can be molded to accurate dimensions can damage the mating members.
If the mating member is made of a metallic material of relatively low hardness, it may be damaged by high-hardness fibrous reinforcing material such as glass fiber, and if the mating member is damaged even slightly and the sliding surface becomes rough, wear speed is accelerated quickly.
In particular, for precision sliding parts of electronic devices made of synthetic resin, when they begin to slide not smoothly, they may have bad influence on the function of the electronic machines or devices. For example, if the sliding properties of a carriage of a computer printer or scanner lower, disturbance may occur in read or reproduced images or noise may be produced due to increase in mechanical sound.
An object of this invention is to provide precision sliding parts made of a resin composition which have required precision molding properties and dimensional stability, which are low in friction coefficient, stable in sliding properties and superior in wear resistance, and which allow use without grease.
According to this invention, there is provided a resin composition for precision sliding parts, the resin composition comprising a modified polyphenylene ether resin as a main component, a fibrous reinforcing agent, and a solid lubricant comprising tetrafluoroethyelene resin and graphite in the amount of 2-30 volume %.
Such a resin composition according to the present invention has good heat resistance, fire retardancy and injection moldability. Because of low specific weight, the end product is light weight. Also, the fibrous reinforcing agent added gives good dimensional stability.
The modified polyphenylene ether resin used in the present invention is an alloy of polyphenylene ether with one or more selected from a noncrystalline polystyrene resin, a crystalline polystyrene resin such as syndiotactic polystylene resin and a polyamide resin. Such a resin is free from shrinkage or deformation upon crystallization and excellent in dimensional stability.
Because both tetrafluoroethylene resin and graphite are used as solid lubricants, the resin composition according to the present invention can form a transfer film of low friction coefficient on the mating member and exhibit good slidability and wear resistance not obtained with tetrafluoroethylene resin only.
The resin composition according to the present invention has both stably low friction coefficient and excellent wear resistance because it contains tetrafluoroethylene resin and graphite in the amount of 2 to 30 volume %. These properties do not impair the fire retardancy and moldability of modified polyphenylene ether (PPE).
The fibers dispersed in the sliding part increase the contact area of the fiber ends with the mating member. This reduces damage to the mating member by fibers and minimizes the amount of wear of the sliding surface.
Though dispersed in the resin, the fibrous reinforcing material used in this invention reinforces the lubricative resin composition. The precision sliding part thus reinforced meets the required precision moldability and exhibits low friction coefficient.